Generally, light-emitting diodes (LEDs) are formed using a diode with a first contact layer, an active layer, and a second contact layer on a substrate. When these layers are forward biased, light at various wavelengths can be generated, which then propagates outward in multiple directions. Some of these directions may be undesired, such as when the light is intended to propagate in a particular direction, or the light impacts a light-absorbing material (such as a silicon substrate), which decreases the overall luminosity of the LED.
These problems of luminosity and directionality have been treated by such methods as placing a distributed Bragg reflector (DBR) beneath the active layer of the LED to reflect the light away from the substrate. However, DBRs become less efficient reflectors at higher incident angles relative to normal to the surface of the DBRs. Additionally, some light may not even impact a DBR as the light escapes the LEDs and propagates toward the substrate, which may absorb the light and prevent its reflection.
Another solution has been to use a light transmissive substrate such as sapphire and form a reflective layer on an opposite side of the substrate from the LEDs. However, this procedure limits the materials available for LED substrates to light transmissive substrates such as sapphire, and prevents the usage of more preferred substrates such as silicon. This can lead to more complicated and expensive manufacturing processes.
Accordingly, what is needed is a structure that allows for the use of more materials for substrates without a corresponding absorption of light.